This invention relates an arrangement for load transfer, especially of petroleum fluids, between two vessels at sea, comprising at least one bow hawser and at least one loading hose arranged to be drawn between the stern of a first vessel and the bow of a second vessel.
Load transfer between two vessel units at sea is a very demanding task, especially due to frequently occurring relative movements between the vessels. A known method to perform this kind of operations is to keep the vessels in a tandem arrangement, i.e. that one of the vessels is kept moored from its bow to the stern of the other vessel. Typically the latter of these may be a so called FPSO (Floating Production, Storage and Off-loading), while the first mentioned may be a tanker, especially a shuttle tanker. Normally one single nylon hawser is used for the moorage, because such hawsers are rated for the large forces which may occur, which depends primarily on the tonnage of the vessels. A typical break load for the nylon hawser may be about 550 tonnes. To use nylon hawsers is advantageous because that material gives a certain elasticity or spring effect by relative movement between the two vessels. The vessel separation may be about 50 to 90 meters, whereas a relatively narrow separation is adjusted to, if a stiffer connection is wished, while a wider separation arranged will result in a more flexible connection or moorage.
Obviously such operations entail large strains under bad weather conditions with wind and waves. The two vessels in such a tandem arrangement may move relative to each other both in the longship and athwart directions. With conventional moorage methods, experience shows that it is needed to use advanced and expensive dynamic positioning systems, especially for shuttle tankers, with its bow moored to the stern of an FPSO-vessel. Despite the known precautions taken in such an arrangement, it happens that the vessels touch each other or even collide. This of course implies a considerable risk with regard to safety and economy.
By a method according to the above mentioned, with a more or less conventional tanker is going to receive a fluid load from an FPSO, the loading hose is led from the stern of the FPSO and back along the side of the hull of the tanker in order to be connected to a manifold or similar equipment which in the usual manner is to be found near midship of the tanker vessel. However, arrangements are also known with the loading hose led from the stern of the stationary FPSO-vessel to the bow of a tanker, e.g. an especially adapted shuttle tanker, which advantageously may carry advanced equipment for dynamic positioning for this purpose. An other possibility is that the tanker vessel is assisted by a tender at its stern, in order to keep the tanker vessel at safe distance from the stern of the FPSO-vessel. The FPSO-vessel may also carry a loading hose carrying boom reaching out over the stern.
On the background of the known art described above, the invention concerns an arrangement for load transfer between the stern of a first vessel and the bow of a second vessel, comprising two bow hawsers arranged to run from each their anchor points near the sides of the ship at each side of the stern, to the bow, with approximately the same length of the two hawsers, and that the loading hose is arranged to run essentially centrally from the middle point of the stern, to the bow.
In this way a mooring is achieved with the mentioned second vessel, e.g. a shuttle tanker, with its bow position held nearly fixed with respect to the centre line of the first vessel, such as an FSO- or an FPSO-vessel. The hawsers and the loading hose of the preferred embodiment will be stored on board of the FSO/FPSO-vessel.
Among the achieved advantages with such a solution is especially a strongly improved safety of the actual operations together with a reduced demand for advanced dynamic positioning equipment. Further, the effect of weather and wind waves will be less than in the known arrangements and operations, partly due to that the first vessel will act as a breakwave for the second vessel.
The advantageous moorage conditions achieved obviously depend on the relative length dimensions, that is the basic triangular geometry of the arrangement. Clearly the hawsers cannot have a length shorter than a certain minimum because this would bring the vessels too close to each other. On the other hand, too long lengths of the hawsers would imply that the steering or centering effect on the mentioned bow in relation to the centerline of the stationary vessel would be weak. Consequently, according to the invention, it is to prefer that the lengths of the hawsers are of comparable length with the mutual distance with the anchor points on each side of the stern of the first vessel. This may imply that the hawsers"" lengths are up to 1.5 or even 2 times the mentioned mutual distance. In this connection it is worth while noting that the first vessel, such as an FPSO, normally is of considerable dimensions, and thus with a corresponding width of the stern.
As will emerge from the following, it may, for certain conditions and operations, be safeguarded considerable advantages by using two pairs of hawsers and/or by arranging extension devices for the hawsers, in order that the hawsers be of increased effective length while keeping a reasonable mutual separation between the vessels.
The solutions comprised by the inventions may be adapted for several different areas of use and operations, such as:
A more or less stationary vessel of the FSO- or FPSO-type situated in areas with high risk weather, eventually under milder weather conditions.
That the load to be transferred is petroleum fluids.
Transfer of liquid gas (LNG, LPG, NGL) and other corresponding or demanding products.
In this connection it shall be noted that especially by the above mentioned known tandem operations by two vessels under conditions or in situations as mentioned in the previous passage, the known moorage methods often imply considerable problems, especially about relative movements of the two vessels with regard to athwart, longship and rotational movements.
Such undesirable movements will by means of the arrangement according to the invention be stabilized to a considerable degree, which may be supported by ensuring a continuous even drag astern from the other [trailing] vessel which bow is connected with the leading vessel.